The present disclosure relates to a collimator lens having a temperature compensating function, an optical scanning device which uses the foregoing collimator lens, and an image forming apparatus which uses the foregoing optical scanning device.
For example, a general optical scanning device that is used in laser printers, copiers and the like includes optical components including a light source which emits a laser beam, a polygon mirror which reflects and deflects/scans the laser beam, an incident optical system which causes the laser beam to enter the polygon mirror, and a scanning lens which images the deflected/scanned laser beam on a peripheral face (surface to be scanned) of a photoreceptor drum. The incident optical system includes a collimator lens which converts the diverging laser beam into parallel light or convergent light, and a cylindrical lens which converts the parallel light or the convergent light into linear light and images the linear light on a reflecting surface of the polygon mirror. These optical components are generally housed in a resin housing for dust prevention.
Meanwhile, when the temperature of the environment that the optical scanning device is disposed changes, the optical performance of the optical components is affected. In particular, when lens components made from a resin material are used, the influence is significant. When the ambient temperature changes, for example, variation in the refractive index of the lens component, variation in the emission wavelength based on the temperature characteristics of the laser diode which emits the laser beam, change in the lens shape caused by thermal deformation and so on will occur. Moreover, due to the thermal deformation of the housing, the distance to the photoreceptor drum or the distance between the optical components will change. Particularly, since a collimator lens has high error sensitivity, if it is molded from a resin material in which the refractive index changes easily due to the temperature, the imaging position tends to change as a result of the temperature change. Accordingly, it is necessary to provide some kind of temperature compensating function.
Known is technology where, as a result of adding a diffractive-optical element to one face of the collimator lens, change in power associated with the variation in the refractive index induced by the change in the ambient temperature, and variation in the imaging position caused by the mode hopping of the laser diode can be compensated. In order to achieve this fluctuation compensation, various parameters such as the focal distance of the scanning lens and spot diameter of the main scanning cross section are defined with a relational expression.
Nevertheless, there are cases where the imaging performance cannot be sufficiently compensated merely by compensating the fluctuation of the imaging position. In other words, just because the imaging position is adjusted, it does not necessarily mean that the imaging performance such as aberration or the like will also be favorably adjusted. Moreover, it is also necessary to give consideration to measures against the thermal expansion of the housing. Accordingly, even when attempting to configure the housing using relatively inexpensive materials with a large linear expansion coefficient, there was a problem in that, since the imaging performance is insufficient, such inexpensive materials could not be used.
An object of the present disclosure is to provide a collimator lens having a temperature compensating function having superior imaging performance, an optical scanning device which uses the foregoing collimator lens, and an image forming apparatus which uses the foregoing optical scanning device.